The general organization of structural genes and transcriptional sites of heterogeneous nuclear RNA (HnRNA) in man will be studied using an in situ hybridization technique which allows the simultaneous visualization of the radioactive probe and high resolution G-banded prophase chromosomes. 125I-labeled polysomal poly(A)-rich mRNA isolated from HeLa cells will be hybridized under conditions allowing the detection of multiple and single copy genes. To localize sequences coding for HnRNA, use will be made of 125I-labeled double stranded regions of HnRNA from HeLa cells. In addition, 3H-cRNA to two sea urchin (S. purpuratus) histone gene segments, one coding for H1, H2B, and H4 and the other for H2A and H3, which have been cloned in an E. coli plasmid, and a recombinant lambda genome carrying the mouse beta-globin gene, labeled with 3H by nick-translation, will be hybridized to prophase chromosomes in order to localize specific structural genes at a refined level. Total mRNA, the beta-globin sequence, and cRNA to histone genes will be localized to chimpanzee and gorilla G-banded prophase chromosomes to compare the general organization of structural genes and of specific gene sequences between these two closely related species and man.